The brake system of this invention comprises a hydraulically operated, caliper type brake intended for a very special application; it is to be positioned over the brake disc which is mounted on the output shaft of a fluid drive, between the fluid drive and a boiler feed water pump of the type used in connection with the boiler of an electric power plant.
Conventionally, a separate hub is mounted on the output shaft with a shrink fit, and the brake disc is keyed or bolted to the hub. Such arrangements usually cause the shaft to bow, causing imbalance and resultant vibration when rotating.
Conventional brake systems are generally of one of two designs. The first is an electro-magnetic brake wherein the stationary components completely surround the shaft. In this case, the output shaft and the entire magnetic brake mechanism must be removed from the fluid drive to do inspection and/or maintenance, on either the output shaft or the brake. The second is an air over hydraulic design which has fixed uprights with a removable yoke, for the manually operated mechanical brake. The shaft with the brake disc can be removed after removal of the yoke. However, inspection and/or replacement of the calipers is difficult because the vertical uprights are fixed. Also, in such a conventional system, a multiplicity of hydraulic brake actuators and pads, for example, four, are mounted on the fixed upright or stanchion, the pads being arranged in an arc, generally along the lower half of the brake disc. This tends to induce lateral forces on the disc and shaft, hence, on the bearings of the output shaft.
An example of conventional air over hydraulic device is shown in FIG. 3, where a complete device 130 is shown as having a fixed, one-piece stanchion 131 with two upright arms 132 and a lower, central reach 133 with an arcuate upper surface 134. The stanchion 121 is fixedly mounted on a base 135. Four caliper brakes 136, mounted on the stanchion 131, are spaced in a semicircle around the bottom half of a brake disc 137 bolted to a hub 145 which is mounted with a shrink fit to an output shaft 146. The calipers 136 are supplied with hydraulic fluid from a master cylinder 138, connected by fluid line 139 that also communicates with an accumulator 140. The individual calipers are connected serially or in parallel, or both, by a connecting fluid line 141 to the line 139. In this arrangement, if there is a leak in the line 141 or 139, the master cylinder acts once, and if insufficient pressure is brought to the calipers, the brake cannot stop, or cannot continue to hold the disc stopped, and hence the brake will fail.
The purpose for having a brake on the output shaft of a fluid drive is to aid in the operation of the boiler feed pump by stopping the boiler feed pump shaft and to keep it from rotating when the pump is out of service, but when the driver, usually a steam turbine-generator, remains in service. There are certain boiler feed pumps that are designed and built in ways that will cause them to gall and consequently to seize if they rotate for more than a few seconds or a few minutes at low speed with no or low flow of water through them. Typically, these pumps have stainless steel components that can rub together. They are often, although not always, high performance, high pressure pumps.
Continued use of a boiler feed pump over a period of years permits the operators to establish if the pump has seized at low speed when the pump was out of service and a brake was not being applied. For those pumps that either have seized under low speed and flow conditions, or for which the manufacturer advises against operation at low speeds and low flow, it is imperative that a brake be employed when the driver is a steam turbine-generator. For those boiler feed pumps which have not seized when they were operated at low speed for several hours, or for which the manufacturer indicates that no problem will arise by operating at low speed with low to no flow, or which, generally, are motor driven, the brake is not required.
The speed and developed horsepower of the output shaft of a fluid drive depends upon the position of the scoop tube and the amount of circuit oil flow. This is discussed at some length in a co-pending application of Melbourne F. Giberson, Ser. No. 07/998,959, and broadly speaking, is well known in the art.
Usually, the hydraulic and mechanical brake systems in use presently can stop and hold the output shaft/coupling/boiler feed pump if the following conditions are met:
1) the scoop tube tip clearances are properly set; PA0 2) the scoop tube is intact, not broken off: PA0 3) the scoop tube linkage internal and external to the fluid drive is intact; PA0 4) the scoop tube is kept at the minimum power position, i.e. at its maximum radial reach; PA0 5) the circuit oil flow is at a minimum; PA0 6) no vane is broken at either the impeller or runner; PA0 7) the gap between the impeller and runner is properly set; PA0 8) the journal and thrust bearings are not worn to the point at which the impeller and runner are touching, either axially or radially; PA0 9) the brake disc is intact; PA0 10) the calipers are not excessively worn; PA0 11) the hydraulic brake system is not leaking oil; and PA0 12) the hydraulic brake is designed to stop the shaft and to keep the shaft from rotating, assuming that the control power remains on.
Conventionally, a mechanical pin, usually made of high strength alloy steel, is inserted through a hole in the brake disc and through one of the slots in a holding block fixed against rotation with the disc. However, the pin, like the brake itself, is not designed to withstand the torque of the turbine shaft of a large turbine. For example, in a three hundred thirty megawatt unit at full load, the available power of the output shaft is four hundred forty thousand horse power; for a seven hundred fifty megawatt unit, the available power is a million horse power. Clearly, a pin or a caliper type brake is not going to keep the shaft from rotating against such torque.
Historically, for those fluid drives that have brakes, the brake is one of the highest maintenance items; they cause many of the forced outages. The brakes fail for a variety of reasons, usually when one or more of the conditions listed above is violated, with the result that the brake is not able to keep the shaft from rotating.
The fluid drive has first priority on the steam power, ahead of the generator, regardless of whether the fluid drive is on the turbine end or on the generator end of the machine.
Reported failures of brakes are innumerable. Pins have been sheared, calipers worn out in a few seconds, brake discs glow red hot, brake discs are broken and scattered. Accordingly, if the brake does not need to be applied in order to prevent the pump from galling and seizing, then the brake should be removed. If the brake does need to be applied in order to prevent the pump from galling and seizing, then the brake should be used, but used sparingly and kept in excellent condition.
As has been indicated, the brake system does not provide assurance of safety for maintenance of any form, no matter how simple, no matter how short in duration. The only purpose for which the brake is installed is to aid in the operation of the boiler feed pump.
In the brake systems known heretofore, the brakes have been configured and installed in such a way that the brakes have had to be disassembled either for maintenance of the brakes or for removal of the output shaft from the fluid drive with the brake disc on it.
One of the objects of this invention is to provide an improved hydraulic brake system that is more reliable than brake systems now in use;
Another object is to provide a brake system in which the brake elements are more easily disassembled in order to service the calipers or pads of the brake, or to remove the output shaft of a fluid drive.
Another object is to provide an improved hydraulic caliper brake system in which vibration and lateral forces are minimized;
Other objects will become apparent to those skilled in the art in the light of the following disclosure and accompanying drawing.